Key differences between Copy Constructor in C++ and Assignment Operator in C++

Copy Constructor in C++

 In C++, a copy constructor is a special constructor used to create a new object as a copy of an existing object. Whenever an object is declared and initialized using another object of the same class, the copy constructor is invoked to handle this initialization. The typical syntax for a copy constructor takes a reference to an object of the same class as its parameter, usually as a constant reference. The purpose of a copy constructor is to ensure that the new object precisely replicates the state of the object from which it is being copied. This includes deep copying of data that involves pointers, thus preventing issues such as shallow copying where the new and original objects might inadvertently share the same dynamically allocated memory, leading to potential errors and data corruption.

Functions of Copy Constructor in C++:

  • Object Initialization:

Enables the initialization of one object from another of the same type, ensuring the newly created object has the same state as the source object.

  • Copy Semantics:

Provides precise control over how objects of a class are copied, which is crucial when the class involves resource management, like dynamic memory or file handles.

  • Pass-by-Value:

Facilitates the passing of objects by value to functions. Without a properly defined copy constructor, passing objects by value might lead to shallow copies, which can cause errors.

  • Return-by-Value:

Enables objects to be returned by value from functions. The copy constructor ensures that the temporary object created as the return value is properly initialized.

  • Exception Safety:

Helps in providing strong exception safety guarantees by ensuring that objects can be copied safely during exception handling without affecting the state of the original object.

  • Managing Resources:

Ensures deep copying of dynamically allocated resources, preventing issues like double free errors and memory leaks that can arise with shallow copying.

  • Array Initialization:

Allows for the initialization of arrays of objects where each object needs to be initialized with the state of an existing object.

  • STL Containers Compatibility:

Ensures compatibility with Standard Template Library (STL) containers that require copy semantics for elements stored within containers like vector, list, and deque.

Example of Copy Constructor in C++:

This example includes a class named Box which encapsulates the concept of a box having width, height, and length attributes. The copy constructor is used to create a duplicate object with the same dimensions as the original.

#include <iostream>

class Box {


    double width, height, length;

    // Default Constructor

    Box() : width(0), height(0), length(0) {}

    // Parameterized Constructor

    Box(double w, double h, double l) : width(w), height(h), length(l) {}

    // Copy Constructor

    Box(const Box& b) {

        width = b.width;

        height = b.height;

        length = b.length;

        std::cout << “Copy constructor called.” << std::endl;


    // A method to calculate volume

    double volume() const {

        return width * height * length;



int main() {

    // Create a box object

    Box box1(3.0, 4.0, 5.0);

    std::cout << “Volume of box1: ” << box1.volume() << std::endl;

    // Create a copy of box1

    Box box2 = box1;  // This will call the copy constructor

    std::cout << “Volume of box2: ” << box2.volume() << std::endl;

    return 0;


Output Explanation:

  • The program defines a class Box with a default constructor, a parameterized constructor, and a copy constructor.
  • The box1 object is initialized using the parameterized constructor.
  • The box2 object is initialized as a copy of box1, invoking the copy constructor. The copy constructor outputs a message indicating its invocation and copies the attributes from box1 to box2.
  • Finally, the program prints the volume of both box1 and box2, confirming that box2 was properly initialized with the same dimensions as box1.

Assignment Operator in C++

In C++, the assignment operator (=) is used to copy values from one object to another already existing object. Unlike the copy constructor that initializes new objects with values from another object, the assignment operator is called when an existing object is assigned a new value from another existing object. This operator can be overloaded to handle specific actions beyond simple member-wise copying, such as deep copying complex objects which contain dynamically allocated memory. Overloading the assignment operator is crucial in classes with pointer members to ensure a proper copy, avoiding shallow copies and potential runtime errors. This allows developers to control how objects of a class are assigned, ensuring resource management and consistency.

Functions of Assignment Operator in C++:

  • Assigning Values:

It assigns values from one object to another existing object.

  • Resource Management:

It manages resources when objects contain dynamically allocated memory, ensuring proper allocation and deallocation.

  • Preventing Memory Leaks:

By correctly handling the copying of resources, the assignment operator can prevent memory leaks.

  • Deep Copying:

Enables deep copying of objects that contain pointers, ensuring that each object has its own copy of dynamic resources.

  • Overloading:

Can be overloaded to customize assignment behavior, which is essential for classes that handle complex states or resources.

  • Chain Assignments:

Supports chain assignments (e.g., a = b = c;), which are useful for initializing multiple objects at once.

  • Improve Efficiency:

Optimizes performance by handling self-assignment safely and efficiently.

  • Consistency:

Maintains consistency in data state among objects, ensuring that the assigned object precisely replicates the state of the source object.

Example of Assignment Operator in C++:

Here’s an example in C++ that demonstrates how to overload the assignment operator for a class that manages a dynamically allocated array. This example highlights the necessity of handling deep copying correctly:

#include <iostream>

#include <cstring> // for strcpy and strlen

class String {


    char* data;


    // Constructor

    String(const char* str = nullptr) {

        if (str) {

            data = new char[strlen(str) + 1];

            strcpy(data, str);

        } else {

            data = new char[1];

            *data = ‘\0’;



    // Destructor

    ~String() {

        delete[] data;


    // Copy constructor

    String(const String& other) {

        data = new char[strlen( + 1];



    // Assignment operator

    String& operator=(const String& other) {

        if (this != &other) { // Avoid self-assignment

            delete[] data; // Free existing resource

            data = new char[strlen( + 1]; // Allocate new memory

            strcpy(data,; // Copy the data


        return *this;


    // Function to display the string

    void display() const {

        std::cout << data << std::endl;



int main() {

    String str1(“Hello”);

    String str2 = str1; // Copy constructor is called

    String str3;

    str3 = str2; // Assignment operator is called

    std::cout << “str1: “;


    std::cout << “str2: “;


    std::cout << “str3: “;


    return 0;


In this example:

  • Constructor initializes a string from a C-string.
  • Copy Constructor creates a new string object from another existing string object, performing a deep copy.
  • Assignment Operator handles assigning one existing string object to another, including self-assignment check, resource deallocation, and memory allocation for a deep copy.
  • Each function ensures that the object manages its memory safely, preventing leaks and errors.

Key differences between Copy Constructor in C++ and Assignment Operator in C++

Aspect Copy Constructor Assignment Operator
Purpose Initialize new object Assign to existing object
Invocation Time Object creation Any time after creation
Parameters Passed by reference Passed by reference
Return Type None Object reference
Self-Assignment Handling Not applicable Needs checking
Memory Allocation Always allocates Conditionally allocates
Overloading Cannot be overloaded Can be overloaded
Required by Compiler Sometimes synthesized Never synthesized
Syntax Constructor syntax Operator syntax
Used for Copying state initially Updating existing state
Default Implementation Shallow copy Shallow copy
Can be implicit Yes No
Must handle freeing memory No Yes
Can be declared private Yes Yes
Common Errors Fewer (often memory init) Many (e.g., self-assignment, leaks)

Key Similarities between Copy Constructor in C++ and Assignment Operator in C++

  • Purpose of Cloning Objects:

Both are used to manage the copying of objects. While the contexts differ (initialization vs. assignment), the end goal is to replicate an object’s state.

  • Handling Object State:

Both typically involve copying the values of all object attributes from one object to another. This can include simple data fields and pointers to dynamically allocated memory.

  • Use of Reference Parameters:

Both the copy constructor and assignment operator usually take their parameters by reference to avoid unnecessary copying.

  • Default Behavior:

In the absence of a user-defined copy constructor or assignment operator, the compiler provides a default version for each that performs a shallow copy of the object.

  • Potential for Custom Behavior:

Both can be overridden to provide deep copying or to handle other complex behavior, such as copying resources like file handles or network connections that require special handling.

  • Influence on Class Design:

The definition and behavior of both the copy constructor and assignment operator can significantly influence the design and efficiency of a class, especially concerning resource management.

  • Rule of Three:

In C++, the existence of a custom copy constructor often implies the need to also define a custom assignment operator and destructor to manage resources correctly, aligning with the Rule of Three.

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